Titanium - Don't even think about using it if it has a bearing surface like a conrod.

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edholly

Sydney Australia
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As per post below - made a conrod for a 1.5cc model engine and this is how it ended up - with only ever a few pops and maybe a few multiple rotations - but - no running.

Total time in service would be equivalent to around a half minute.

As you can see it has galled badly in the extreme, even left the crankpin in such a rough state you could use it for a file ! A black mess of what was probably Ti dust was inside the engine, but thankfully washed out without any damage to other componentry.

When a conventional alloy conrod was made the engine fired and ran within a few flicks, but the friction from what was happening here obviously denied it the ability to start.

So - what seemed like a good idea at the time - wasn't. !!

Obviously if a nice bronze bearing surface was inlaid both ends of the Ti rod, then that would work very nicely - but for now an alloy one serves the purpose as it has in many thousands of these little motors.

Hopefully this might help someone that is thinking along these lines - for me a lesson learnt - but that's what I made it for in the first place to learn a bit more about this "miracle" metal.

Ed

Photo of the oval crankpin end of the rod and galling on the right hand end of the pin.

sp53.jpg
 
Hmmmm , maybe the crankpin is not square. Than you may have that problem. Or the crankshaft is not more straight.

Barry
 
Which grade of Ti was used? Titanium rods were popular in RC engines a while back. I don't know if they were all bushed, I wasn't very involved at the time.

Greg
 
Apart from material??, lots of lubrication?? (15% to 20% Oil for run in) and the very close tolerances required for these fits, a circular crossection for the conrod's shaft is a very bad idea. I made a sketch for explanation.

knife-crossectioned-conrod-explained.PNG
 
Apart from material??, lots of lubrication?? (15% to 20% Oil for run in) and the very close tolerances required for these fits, a circular crossection for the conrod's shaft is a very bad idea. I made a sketch for explanation.

yea the ti is pretty stiff, but the material is tolerant of flex. you should be able to use it as a rod with a pretty small cross section. betweens it's strength and memory (assuming it's something like ti6al4v) you shold be able to get it very flexy without fear of breaking. but i probably would bush it as well.

also have you considered tool steel? i have to see if there is info on alloys but there is a tool steel that is replacing titanium as a retainer material for racing engines. it's not light but it's probably easier to heat treat (atleast on a production scale) because it might not need an inert envirmonment and despite the density difference the steel is so strong as far as tensile strength they can slim it down to the point that they are lighter than the titanium alternatives. they are basically sheet metal thin in areas and hold up great. as thin as these things are they must be flexing so they must have decent fatigue characteristics.
 
It's not the overall size of the crossection or specific stiffness, the problem is the shape (geometry). The elastic section modulus about the axis is the interesting point.

What we want is great strenght in direction of the swiveling motion (high section modulus), and some flex about the other axis to compensate for angular missalignment caused by the flex of the crankshaft.

A circular cross-section has the same sectional modulus about every interesting axis (because it's round ;)), so it can't meet our requirements.

Our requirements call for a rectangular cross-section, forming a flat shaft. In German these are called Messerpleuel. say: [MessRployal]. Messer = knife. Pleuel = conrod.
 
Guys,

Thanks for the replies ....

The grade of Ti is labelled Ti Gr2 0 Bar and another spec written on it which might be the supplier's stock number HT=T91226.

I looked up Grade 2 on various sites and it is one of the "softer" grades with little alloying of other metals, nevertheless it should be strong - which no doubt it is.

But it would appear that as a model aircraft engine conrod where the ends are unbushed, then it cannot cope with the friction of the rotational forces as can aluminum or bronze, or for that matter steel, and galls terribly.

My fuel was standard 1/3 ether, kero and castor oil.

I think the fact that the crankpin is impregnated with it, or abraded so badly by it, reinforces this theory. The conrod end of the pin is like its been impregnated with 1200 grit !

Certainly nothing else was changed other than removing the Ti rod and replacing with an Al one and replacing the crankpin with a new one and the engine is now as happy as any other engine with an alloy rod.

I am not unhappy that the Ti one failed - the main reason for going down this route was to see how it fared. If we don't sometimes experiment with the things that we would like to do, then we don't learn from our mistakes ! Imagine making a Ti crankshaft and then finding it lasted 30 seconds - now that would be a bit more of an Ouch !

Ed
 
TI like SS is known to work harden without notice. It also is very prone to galling and self welding. Reasons that nuts are made of different alloy than bolts is to prevent self welding.
 
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